CO2 geological storage is about pumping a reactive fluid underground and ensuring it doesn't find a way back to the atmosphere for a very long time - possibly centuries. Potable aquifers and other permeable formations (e.g. hydrocarbon deposits) must also be protected against CO2 contamination.
Wells are generally recognized as a weak spot in CO2 storage, where containment can break down. This is because cement, steel and elastomers can be corroded by CO2, and the ageing process will be accelerated by any defects in the cement sheath.
It is therefore of critical importance to understand and characterize fluids and solids across the caprock. This has the triple aim of: verifying the soundness of the complex cementing engineering process, evaluating the capacity of cement to provide short-term zonal isolation, and providing measures that can be used to predict the evolution of cement and casing over the long term.
This paper will focus on an in-depth evaluation of the annular material on the Otway CRC-1 well that is being used to inject CO2 in the CO2CRC pilot geological storage project. The evaluation will draw on the design and job data, and on a detailed analysis of the high-resolution 3D cement imaging log to characterize the cement and ensure the long-term risk of containment breach is minimized.
The essentially unpredictable nature of fault-free risk - i.e. the unplanned events in a job otherwise designed and executed to the highest standards - always requires a number of mitigation measures to minimize the residual risk of CO2 leaks. This paper will show that the adoption of a number of them on Otway (e.g. excess volumes and CO2-resistant cement system) have been essential in achieving the containment objectives.
Cement slurries are exposed to a number of phenomena during mixing and placement that can lead to set cement properties that are very different from their design value. Density control problems (both for continuous and batch mixing), contamination, channeling and fluid loss can and do cause slurry dilution/concentration and chemical incompatibility, which in turn can have a major negative effect on the capacity of cement to guarantee hydraulic isolation and can even lead to premature gelling during placement and early job termination.
It is currently hotly debated whether ten or more meters of competent cement, well bonded to casing and formation would degrade during the expected isolation timeframe for CO2 geological sequestration wells (1,000's to 10,000 years). This is because competent cement, although reactive if exposed to CO2, has a very low permeability of the order of 0.5 to 5 µD; this low permeability means that most CO2 will travel by diffusion, a very slow process over the length scale of a meter. Cement with a high w/c ratio, however, could have a much higher permeability, less resistance to CO2 aggression and more frequent defects related to slurry settling. Defects such as liquid channels in cement can even provide direct pathways for CO2 leaks that couldn't possibly be healed by calcite precipitation during the CO2 attack.
Some of the phenomena listed above can be predicted, but cannot easily be controlled; others (such as fluid loss) can hardly be predicted at all. In any case they belong to the class of fault-free risk, sometimes called residual risk: events causing sub-standard system performance that cannot be engineered away and that may happen even when job is perfectly executed. Mitigation measures must be adopted in this case to ensure a robust design. This is especially true for wells entering CO2 storage reservoirs, where storage containment is a key performance factor and CO2-cement reactions may cause leaks to grow over time.